1. Field of the Invention.
The present invention relates to a heating and cooling apparatus for heating an environmental or process load by removing thermal energy from the earth and transferring that energy to the load and, similarly, cooling the load by removing thermal energy therefrom and transferring that energy to the earth for dissipation therein.
2. Description of the Related Art.
With the steadily increasing costs of fossil and other depletable types of fuels, which are presently being used to obtain desirable temperature levels in environmental and process loads, greater emphasis is being directed toward developing systems and methods to extract energy from the vast, virtually unlimited thermal energy stored in the earth and transferring that energy to the loads for heating purposes and, reversely, extracting thermal energy from the loads and transferring that energy to the earth for dissipation therein for cooling purposes. One type of previous system for accomplishing such objectives is commonly referred to as a heat pump.
Approaches which have previously been applied on a small scale to accomplish the transfer of useful thermal energy to and from the earth have involved the use of subterranean pipes in flow communication with various above ground devices. A refrigerant coolant, which is pumped through such pipes by a compressor, serves as a carrier to convey thermal energy absorbed from the ground or earth, as a heat source, to the above ground devices for further distribution as desired for heating purposes. Similarly, the coolant carries thermal energy from the above ground devices to the subterranean pipes for dissipation of heat energy into the earth, as a heat sink, for cooling purposes.
In order to simplify installation of such subterranean pipes and to minimize installation costs, the generally preferred configuration of such subterranean pipes is one where such pipes are substantially horizontally oriented. Unfortunately, several complications can arise when the subterranean pipes are installed horizontally.
First, lubricant oil may escape from the compressor while the system is operating. The lubricant oil is carried along with the refrigerant throughout the system. Because of the lower elevation of the subterranean pipes, the lubricant oil tends to accumulate therein. As a result, the compressor is gradually deprived of essential lubricant oil, which jeopardizes the continued successful operation of the compressor. Further, the accumulation of the lubicant oil in the subterranean pipes gradually floods those pipes, substantially reducing the ability of the subterranean pipes to perform their originally intended function.
This problem basically arises from a conflict between two potentially adverse objectives: to maintain a relatively low rate of refrigerant flow through the subterranean pipes whereby the refrigerant can substantially undergo a complete change of phase while transmitting the subterranean pipes as opposed to generating sufficient refrigerant flow velocity through the subterranean pipes to sweep the oil along with the refrigerant to avoid accumulation of the refrigerant oil in the subterranean pipes. Previous attempts to accordingly control the refrigerant flow have included complicated refrigerant distribution configurations.
Second, when an energy demand cycle is completed, the system shuts down to wait for a subsequent demand for energy transfer. As a result, a certain amount of liquid refrigerant then passing through the subterranean pipes will lose its momentum and will remain in the subterranean pipes. When the subsequent energy demand occurs, the compressor, which is generally designed for pumping gas as opposed to pumping liquid, quickly depletes the gaseous refrigerant trapped between the liquid refrigerant, which remains in the subterranean pipes, and the compressor such that a low pressure condition is quickly created at the input of the compressor. Most compressors are generally designed to interpret such a low pressure condition at the input as an indication that insufficient refrigerant exists in the system to properly function. As a result, the compressor automatically shuts down when such a low pressure condition is sensed in order to protect against burn-out of the compressor due to the possible absence of sufficient refrigerant.
A similar but more pronounced low pressure problem is encountered when a reversible system changes from a heating mode to a cooling mode due to a refrigerant capacity imbalance which is inherent in a reversing system. This imbalance results from the much larger volume capacity of the subterranean heat exchanger as compared to the volume capacity of the dynamic load heat exchanger.
When the operating cycle is reversed, additional time must be allowed to manipulate the excess refrigerant whereby the refrigerant can assume its appropriate redistribution throughout the system in order to properly function in the reverse mode. During this remanipulation time period, the low pressure condition is created at the input of the compressor. The generally relatively short time interval allowed for the low pressure condition at the compressor input before shutdown can be insufficient for the compressor to overcome the inertial resistance of the static refrigerant in the subterranean pipes and redistribute the refrigerant for the reverse mode. As previously described, the low pressure condition at the compressor input can cause the compressor to automatically prematurely shut down.
This imbalance can be particularly troublesome during a system start-up at the end of an extended heating cycle where the temperature of the earth surrounding the subterranean pipes has been reduced as a result of extraction of thermal energy therefrom. As a result, a large volume of refrigerant can accumulate in the pipes of the subterranean heat exchanger.
A third problem, which is generally observed for prior art heat pumps, is the absence of a mechanism for achieving refrigerant pressure equalization subsequent to system shutdown to reduce start-up loads and thereby extend service life of the compressor.
Previous attempts to circumvent some of the aforesaid problems have generally followed either of two approaches: (i) using a plurality of closed loop systems working in combination, with one of such loops horizontally or vertically disposed subterraneally, or (ii) using a vertically disposed, single-closed-loop subterranean exchanger.
The plural loop approach generally utilizes indirect heat exchange rather than direct thermal exchange. This approach basically employs two or more distinct and separate, cooperating, closed loop systems. A first one of such closed loop systems is sequentially routed through the earth and through an interim heat exchanger transferring thermal energy therebetween. The other one of such closed loop systems, which is sequentially routed through the interim heat exchanger and through a dynamic load heat exchanger for further distribution as desired via techniques commonly known in the heating and cooling industry, operably interacts with the first such closed loop system in the interim exchanger. Thus, through the cooperative effort of the two separate closed loop systems, thermal energy is indirectly transferred between the earth and the environmental or process load.
By using a plural loop approach, the oil deprivation problem can be partially resolved by eliminating the refrigerant and oil transitting the subterranean pipes, thereby minimizing the quantity of oil which can be drained away from the compressor or by using a non-phase-change heat transfer fluid in the subterranean portion of such a plural loop system. Such double closed loop systems are substantially more complicated, due to the greater number of components required, and are considerably less efficient than properly designed, single closed loop systems.
The vertical single-closed-loop approach generally utilizes downwardly or vertically inclined subterranean pipes. Such a system can generally be designed to operate in either a heating mode or a cooling mode. Unfortunately, however, the same system will not properly function when operated in a reverse mode due to the disruption caused by the difference in specific density of gaseous refrigerant relative to that of liquid refrigerant. Specifically, while the transition from liquid to gas may be designed to occur while the refrigerant is passing upwardly in one mode of operation, such transition may occur while the refrigerant is passing downwardly in the reverse mode of operation.
Another shortcoming of a vertical loop system is the entrapment of oil at the lower extremities of the vertically oriented pipe, thereby depriving the compressor of essential lubricant oil. In addition, such vertical loop systems require sloped or vertically bored holes for installation of the subterranean pipes; such installations are limited to cooperating sub-soil conditions, such as where borings do not tend to cave-in before the pipe installations can be completed resulting in substantial unnecessary expense.
What is needed is a reversible heating and cooling system having a single closed loop configuration which efficiently and reliably returns lubricant oil to the compressor, which automatically compensates for the fluctuation in quantity of refrigerant which occurs when reversing modes of operation and which anticipates the low pressure condition at the compressor input which arises during system startup and reversal.